Generally, golf balls have been classified as wound balls or solid balls. Wound balls are generally constructed from a liquid or solid center surrounded by tensioned elastomeric material. Wound balls are generally thought of as performance golf balls and have a good resiliency, spin characteristics and feel when struck by a golf club. However, wound balls are generally difficult to manufacture when compared to solid golf balls.
Solid golf balls were initially, two piece balls, i.e., comprising a core and a cover. In order to improve the playing characteristics of the golf ball, more recently developed solid balls are comprised of a core, one or more mantle layers and one or more covers.
In the manufacture of solid golf ball cores, all cores have a slight cure gradient. This is a normal result of the cure conditions, which usually heat from the outside in. The current practice is generally to form a core with the most uniformity for the given construction. If it is desired to create cure gradients having large hardness differences across the gradient, or to use materials that are not typical to the art, generally, it has been necessary to use high molding temperatures for long periods of time (i.e. 175° C. for 30 minutes).
The prior art is comprised of a variety of golf balls that have been designed to provide particular playing characteristics. These characteristics are generally the initial velocity and spin of the golf ball, which can be optimized for various types of players. For instance, certain players prefer a ball that has a high spin rate in order to control and stop the golf ball. Other players prefer a ball that has a low spin rate and high resiliency to maximize distance. Generally, a golf ball having a hard core and a soft cover will have a high spin rate. Conversely, a golf ball having a hard cover and a soft core will have a low spin rate. Golf balls having a hard core and a hard cover generally have very high resiliency for distance, but are hard feeling, and therefore lack “touch” and are difficult to control around the greens. Various prior art references have been directed to adding a mantle layer or second cover layer to improve the playability of solid golf balls.
As indicated above, the spin rate of golf balls is the end result of many variables, softness of the cover in relationship to the inner core or an inner mantle layer is but one of these variables. Spin rate is an important characteristic of golf balls for both skilled and recreational golfers. High spin rate allows the more skilled players, such as PGA professionals and low handicapped players, to maximize control of the golf ball. A high spin rate golf ball is advantageous for an approach shot to the green. The ability to produce and control backspin to stop the ball on the green and side spin to draw or fade the ball substantially improves the player's control over the ball. Hence, the more skilled players generally prefer a golf ball that exhibits high spin rate.
On the other hand, recreational players who cannot intentionally control the spin of the ball generally do not prefer a high spin rate golf ball. For these players, slicing and hooking are the more immediate obstacles. When a club head strikes a ball, an unintentional side spin is often imparted to the ball, which sends the ball off its intended course. The side spin reduces the player's control over the ball, as well as the distance the ball will travel. A golf ball that spins less tends not to drift off-line erratically if the shot is not hit squarely off the club face. The low spin ball will not cure the hook or the slice, but the lower spin will reduce the adverse effects of the side spin. Hence, recreational players prefer a golf ball that exhibits low spin rate.
The prior art teaches of having either a soft or a hard intermediate (inner cover) layer formed about a core to achieve particular performances, as seen above, from a golf ball. A means of achieving these types of performance characteristics without the use of any mantle layer may be attainable through the use of gradient curing to create hardness gradients on the outer skin or shell of the golf ball core.
U.S. Pat. Nos. 5,803,834, 5,733,206, 5,976,443, 6,113,831, 5,697,856, 4,650,193, 4,570,937, and 4,858,924 are examples of creating gradients in the core of a golf ball.